1. Field of the Invention
The present invention relates to the field of mass spectrometry, and more particularly to an ion trap mass spectrometer and an ion trap mass spectrometric method designed to cover a wide mass range in one cycle of MSn analysis.
2. Description of the Related Art
First of all, a qz value of an ion trap in an ion trap mass spectrometer, a m/z (mass-to-charge ratio) value of a target ion to be trapped, and a potential well depth Dz to be sensed by a target ion, will be briefly described.
The qz value of the ion trap is a parameter defined by the following Formula 1:
                              q          z                =                              4            ⁢            zV                                              mr              0              2                        ⁢                          Ω              2                                                          (        1        )                            wherein: z is an electronic charge of a target ion; V is an amplitude of RF applied to the ion trap; m is a mass of the target ion; r0 is an inscribed radius of the ring electrode of ion trap; and Ω is an angular frequency of the RF.        
According to a theoretical result in a stable region, the ion trap has a qz value ranging from zero to 0.908 (if a sinusoidal wave is used as a trapping RF voltage waveform). As to a mass range of trappable ions, given that a m/z value of an ion trappable when qz max=0.908 is a low-mass cutoff (LMCO) value, an ion having a m/z value greater than the LMCO value will be trapped.
The potential well depth Dz to be sensed by a target ion is expressed as the following Formula 2:Dz∝qzV  (2)
The qz value becomes smaller as a mass of trapped ions increases, as seen in the Formula 1. Accordingly, the potential well depth Dz becomes smaller, as seen in the Formula 2 (see the following Table 1). If the potential well depth Dz becomes smaller, an ion trapping efficiency will be lowered. This means that there is an effective upper limit on the mass range of trappable ions.
TABLE 1Relationship of qz value, mass-to-charge ratio m/z and potential welldepth Dzqz valuesmall (zero)→large (qz max)m/zlarge→small (LMCO value)Dzsmall→large
In mass spectrometry, a collision-induced dissociation (CID) process is widely used as a technique for exciting and dissociating a molecular ion in an ion trap. The CID process is designed to accelerate an ion based on resonance excitation so as to dissociate the ion through a collision with an inert gas (e.g., He or Ar). In view of obtaining higher dissociation efficiency, it is necessary to increase the potential well depth Dz to be sensed by a precursor ion (i.e., a target ion to be trapped), so as to allow the precursor ion to have higher kinetic energy. Typically, the LMCO value is set to be ⅓ to ¼ of a mass of a precursor ion. Therefore, a fragment ion having a mass less than ⅓ to ¼ of a mass of the precursor ion cannot be measured (see FIG. 1A).
As an alternative technique to the CID process, there has been known an infrared multiphoton dissociation (IRMPD) process designed to irradiate a molecular ion with high-intensity infrared light so as to vibrationally excite and cleave the molecular ion. As compared with the CID process, the greatest advantage of the IRMPD process is in that the dissociation efficiency is not dependent on the potential well depth Dz, and therefore the LMCO value can be lowered during infrared light irradiation to allow a fragment ion having a relatively small mass to be measured (see FIG. 1B). The IRMPD process is disclosed, for example, in “Ion activation methods for tandem mass spectrometry”, Lekha Sleno and Dietrich A. Volmer, Journal of Mass Spectrometry, 39 (2004), 1091-1112, and “Thermally Assisted Infrared Multiphoton Photodissociation in a Quadrupole Ion Trap”, Anne H. Payne and Gary L. Glish, Analytical Chemistry, 73 (2001), 3542-3548.
Although the IRMPD process theoretically has a wider mass range measurable at once as compared with the CID process, an actual measurable mass range based on conventional ion traps is not sufficiently wide in the existing circumstances. With a view to lowering the LMCO value, the conventional ion trap is designed to reduce an amplitude of a trapping RF sinusoidal voltage waveform while keeping a frequency of the voltage waveform (this type of ion trap will hereinafter be refereed to as “amplitude-driven ion trap”; the amplitude-driven ion trap employs a resonator for generating the sinusoidal voltage waveform, and thereby it is difficult to change the frequency). As a result, the potential well depth Dz to be sensed by a precursor ion or a fragment ion is reduced to cause significant deterioration in ion trap efficiency.